The present invention relates to a catalyst for selective removal through oxidation of carbon monoxide from a gas that consists essentially of hydrogen, to a method for producing the catalyst, and to a method of using the catalyst for producing a hydrogen-containing gas through oxidative removal of carbon monoxide from a carbon monoxide-containing, hydrogen-containing gas.
Fuel cells for power generation do not so much pollute the environment and their energy loss is low. Other advantages are that they can be installed in any desired site, and they are easy to increase, and are easy to handle. Accordingly, fuel cells are specifically noticed these days. Various types of fuel cells are known that differ in the type of fuel and electrolyte for them and in the operating temperature. Hydrogen-oxygen fuel cells (low-temperature-working fuel cells) in which hydrogen serves as a reducing agent (active material) and oxygen (e.g., air) serves as an oxidizing agent have been developed most of all, and will be more and more popularized in future.
Various types of hydrogen-oxygen fuel cells are known that differ in the type of electrolyte and the type of electrode therein. Typical examples are phosphate-type fuel cells, KOH-type fuel cells, and solid polymer-type fuel cells. In these fuel cells, especially those capable of operating at low temperatures such as solid polymer-type fuel cells, platinum (platinum catalyst) is used for the electrodes, and it is easily poisoned with CO (carbon monoxide). Therefore, if CO of higher than a predetermined level is in the fuel for them, the power-generating capability of the fuel cells is lowered. If the CO concentration in the fuel is too high, the fuel cells could not generate power at all, and this is a serious problem.
Therefore, pure hydrogen is preferred for the fuel for these fuel cells having such a platinum-type electrode catalyst. From the practical viewpoint, however, hydrogen-containing gas is generally used for them. This is obtained through steam reforming of various types of ordinary fuels (for example, methane or natural gas (LNG); petroleum gas (LPG) such as propane, butane; various types of hydrocarbon fuels such as naphtha, gasoline, kerosene, gas oil; alcohol fuels such as methanol; town gas, and other fuels for hydrogen production), for which public supply systems have been established. Therefore, a fuel-cell power-generation system equipped with a fuel-reforming unit is now being popularized. However, the reformed gas generally contains a relatively high concentration of CO in addition to hydrogen. Accordingly, it is much desired to develop a technique for converting CO in the reformed gas into CO2 that is harmless to platinum-type electrode catalysts, to thereby reduce the CO concentration in the fuel for fuel cells. For this, it is desirable that the CO concentration in the fuel is lowered generally to at most 100 ppm, preferably to at most 10 ppm.
To solve the problem as above, a technique of utilizing shift reaction of the following formula (1) (aqueous gas shift reaction) has been proposed for reducing the CO concentration in fuel gas (hydrogen-containing reformed gas) for fuel cells.
CO+H2O=CO2+H2xe2x80x83xe2x80x83(1)
However, reducing the CO concentration in fuel gas through only the shift reaction is limited, as the chemical equilibrium in the reaction is limited. In general, therefore, it is difficult to reduce the CO concentration in fuel gas to at most 1% through the shift reaction.
Accordingly, for further reducing the CO concentration in fuel gas, proposed is a method of introducing oxygen or an oxygen-containing gas (e.g., air) into fuel gas to thereby convert CO therein into CO2. However, fuel gas contains a large amount of hydrogen. Therefore, when CO in fuel gas is oxidized, then hydrogen therein is also oxidized, and, after all, the CO concentration in fuel gas could not be satisfactorily reduced.
To solve the problem, a method of using a catalyst for selectively oxidizing only CO will be proposed in the process of introducing oxygen or an oxygen-containing gas into fuel gas so as to oxidize CO therein into CO2.
For CO oxidation catalysts, heretofore known are various catalysts of Pt/alumina, Pt/SnO2, Pt/C, CO/TiO2, hopcalite, and Pd/alumina. However, these catalysts are not well resistant to moisture, and their reaction temperature range is low and narrow. In addition, their selectivity for CO is low. Fuel gas for fuel cells contains only a minor amount of CO in a majority of hydrogen. Therefore, if the catalysts are used for reducing the minor amount of CO in fuel gas to a lowered concentration of at most 10 ppm, a large amount of hydrogen in fuel gas must be sacrificed through oxidation.
Japanese Patent Laid-Open No. 201702/1993 discloses a method for producing a CO-free, hydrogen-containing gas for automobile fuel cells, which comprises selectively removing CO from a hydrogen-rich, CO-containing gas. The catalyst used in this is Rh or Ru held on an alumina carrier, but this is problematic in that it is applicable to only a gas having a low CO concentration.
Japanese Patent Laid-Open No. 258764/1993 discloses a method of processing a methanol-reformed gas (containing 20% by volume of CO, and from 7 to 10% by volume of CO, in addition to hydrogen) with an Fexe2x80x94Cr catalyst to thereby reduce the CO concentration of the gas to 1% by volume, followed by further reducing the CO concentration of the gas through methanation with a catalyst having a catalytic metal component of Rh, Ni or Pd. In the method, CO still remaining in the processed gas is removed through plasma oxidation. The method provides a reformed gas for solid polymer-type fuel cells, and the gas does not poison the platinum catalyst for the electrode in the cells. However, as requiring a plasma generator, the method is problematic in that the reaction apparatus for it shall be large. In addition, the temperature for methanation in the method falls between 150 and 500xc2x0 C. At such a high reaction temperature, not only CO but also CO2 is methanated, and the methanation consumes a large amount of hydrogen in the gas. For these reasons, the method is unsuitable for CO removal from a hydrogen-containing gas for fuel cells.
Japanese Patent Laid-Open No. 131531/1997 discloses a catalyst for removing CO from a hydrogen-containing gas, and the catalyst comprises ruthenium and an alkali metal compound and/or an alkaline earth metal compound held on a titania carrier. However, this discloses nothing about a combination of titania and alumina for the carrier of the catalyst. In addition, this suggests nothing about the fact that the catalyst with a carrier of titania and alumina combined is significantly superior to the catalyst with a carrier of titania or alumina alone.
The present invention has been made in consideration of the above-mentioned viewpoints, and its object is to provide a CO oxidation catalyst which is effective for selectively oxidizing and removing CO from a hydrogen-containing gas in a broad reaction temperature range, especially even at relatively high temperatures; to provide a method for producing the catalyst; and to provide a method of using the catalyst for producing a hydrogen-containing gas, especially for producing a hydrogen-containing gas favorable to fuel cells.
We, the present inventors have assiduously studied, and, as a result, have found that a catalyst of ruthenium held on a carrier of titania and alumina is effective for selectively oxidizing and removing CO from a hydrogen-containing gas in a broad reaction temperature range. On the basis of this finding, we have completed the present invention.
Specifically, the invention is summarized as follows:
(1) A CO oxidation catalyst of ruthenium held on a carrier of titania and alumina.
(2) A CO oxidation catalyst of ruthenium with an alkali metal and/or an alkaline earth metal held on a carrier of titania and alumina.
(3) The CO oxidation catalyst of above (1) or (2), wherein the weight ratio of titania to alumina falls between 0.1/99.9 and 90/10.
(4) The CO oxidation catalyst of above (2) or (3), wherein the alkali metal is at least one selected from potassium, cesium, rubidium, sodium and lithium.
(5) The CO oxidation catalyst of any of above (2) to (4), wherein the alkaline earth metal is at least one selected from barium, calcium, magnesium and strontium.
(6) A method for producing a CO oxidation catalyst of ruthenium with an alkali metal and/or an alkaline earth metal held on a carrier of titania and alumina, which comprises applying a solution of ruthenium and a solution of an alkali metal and/or an alkaline earth metal to the carrier.
(7) The method for producing a CO oxidation catalyst of above (6), wherein a mixed solution of ruthenium and an alkali metal and/or an alkaline earth metal is applied to the carrier.
(8) A method for producing a CO-reduced, hydrogen-containing gas, which comprises selectively oxidizing carbon monoxide in a gas of essentially hydrogen, with oxygen in the presence of the catalyst of any of above (1) to (5) or the catalyst produced in the process of above (6) or (7).
(9) The method for producing a hydrogen-containing gas of above (8), wherein the gas of essentially hydrogen is obtained by reforming or partially oxidizing a hydrogen-producing starting material.
(10) The method for producing a hydrogen-containing gas of above (8) or (9), wherein the hydrogen-containing gas produced is for fuel cells.